Thin film transistor and method of making the same

ABSTRACT

A source electrode (12) and a drain electrode (13) are formed apart on an insulating substrate (11), and a semiconductor layer (14) is formed on the szbstrate (11) between the source and drain electrodes (12, 13). An insulating organic molecular film (21) is formed all over the source and drain electrodes (12, 13) and the semiconductor layer (14). Ions are implanted into a selected top surface region of the insulating oganic molecular film (21) corresponding to the semiconductor layer (14), by which chains of molecules in the surface region to form free carbon, providing a conductive gate electrode (22) and the remaining part of the insulating organic molecular film (21) forming a gate insulating film (23).

BACKGROUND OF THE INVENTION

The present invention relates to a thin film transistor which is used,for example, as a switching element for a picture element electrode inan active liquid crystal display element. The invention also pertains toa method for the manufacture of such a thin film transistor.

A conventional thin film transistor of this kind has such a structure asshown in FIG. 1, in which source and drain electrodes 12 and 13, eachformed by a transparent conductive film, for instance, are disposedapart on a transparent insulating substrate as of glass and an amorphoussilicon or similar semiconductor layer 14 is deposited on the substrate11 between the source and drain electrodes 12 and 13. The semiconductorlayer 14 is covered with a gate insulating film 15, on which a gateelectrode 16 is formed.

The gate insulating film 15 is formed of an inorganic insulator such asSiN_(x) or SiO₂, and the gate electrode 16 is formed of a metallicmaterial such as aluminum.

In general, the gate insulating film 15 of such an inorganic insulatoris formed mostly by a plasma assisted CVD (Chemical Vapor Deposition)process. However, the plasma CVD process introduces difficulty inproducing the semiconductor layer 14 of good quality because its surfaceis bombarded with high-energy particles.

Moreover, since the conventional thin film transistor has the structurein which the gate electrode 16 is protrusively provided on the gateinsulating film 15, the surface of the transistor is relativelyirregular. When such thin film transistors are used in, for example, alight crystal display element, gate buses are likely to be broken by theirregularity of the transistor array. In addition, the conventionalstructure requires, for the formation of the gate electrode 16, twomanufacturing steps of forming a metal layer and then selectivelyetching it away.

As a solution to the problem involved in the formation of the gateinsulating film 15 through the plasma CVD process, there has also beenproposed a thin film transistor of the type employing an insulatingorganic molecular film as the gate insulating film 15. However, thisthin film transistor also uses metal for the gate electrode 16, andhence still calls for the above-mentioned two steps for the formation ofthe gate electrode and suffers its protrusive structure.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a thinfilm transistor which is relatively flat over the entire structureincluding gate, source and drain electrodes.

Another object of the invention is to provide a thin film transistorwhich does not require any etching process for formation of a gateelectrode.

According to the present invention, the gate insulating film of the thinfilm transistor is formed by an insulating organic molecular film and aselected region of the gate insulating film on the opposite side fromthe underlying semiconductor layer is rendered into a conductive layercontaining free carbon, thus providing the gate electrode.

That is to say, according to the present invention, the gate insulatingfilm is formed by an insulating organic molecular film, and through ionimplantation, for example, into a selected region of the gate insulatingfilm, chains of molecules in this region are cut to form free carbon, bywhich conductivity is imparted to the region, obtaining the gateelectrode. In this way, the gate electrode can be obtained with such asimple process and it does not protrude from the surface of the device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view showing a conventional thin filmtransistor;

FIGS. 2A-2E illustrate, in cross-section, a sequence of steps involvedin the manufacture of the thin film transistor of the present invention;and

FIG. 3 is a graph showing the relationship between the amount ofpolyimide ions implanted and the surface resistivity.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring first to FIGS. 2A-2E, an example of the thin film transistorof the present invention will hereinafter be described along with itsmanufacturing method.

The manufacture starts with the preparation of the transparentinsulating substrate 11 as of glass, such as shown in FIG. 2A. Then thesource electrode 12 and the drain electrode 13, each of which is atransparent conductive film of ITO, for example, are formed apart on thesubstrate 11, as depicted in FIG. 2B. After this, the semiconductorlayer 14 of amorphous silicon, for instance, is formed on the substrate11 between the source and drain electrodes 12 and 13 as shown in FIG.2C.

Next, in this embodiment, an insulating organic molecular film 21 ofpolyimide, for example, is deposited all over the surfaces of theelectrodes 12 and 13 and the layer 14 as depicted in FIG. 2D. Theformation of the film 21 can be achieved by use of a spinner coating,offset printing, LB (Langmuir-Blodgett), or like process. The materialfor the insulating organic molecular film 21 need not always be thepolyimide but may also be stearic acid, diacetylene, phthalocyanine,anthracene, or the like.

Next, as shown in FIG. 2E, for example, N⁺ ions are selectivelyimplanted into the top surface of the insulating organic molecular film21, corresponding to the semiconductor layer 14, in an amount of 1×10¹⁷ions/cm² or so with an accelerating energy of 90 KeV, by which chains ofmolecules in the limited upper region of the insulating organicmolecular film 21 are cut to form free carbon and hence provideconductivity therein, forming a gate electrode 22. The thickness, sheetresistance, permeability and work function of the gate electrode 22 aredetermined according to the conditions for the ion implantation. Theintermediate portion of the organic molecular film 21 between the gateelectrode 22 and the semiconductor layer 14 will act as a gateinsulating film 23.

The thicknesses of the gate electrode 22 and the gate insulating film 23are selected in the ranges of, for instance, from 3000 Å to 1 μm andfrom 1000 to 3000 Å, respectively. Accordingly, the thickness of theinsulating organic molecular film 21, which is formed in the step shownin FIG. 2D, is selected substantially in the range of 4000 to 13000 Å.Incidentally, when Ar⁺ ions were implanted into a polyimide film with anacceleration energy of 150 KeV, the relationship between the surfaceresistivity of the film and the amount of ions implanted was such asshown in FIG. 3, from which it appears that the surface resistivityincreases with an increase in the amount of ions implanted. Furthermore,the surface resistivity diminishes as the ion beam current densityincreases.

The formation of the gate electrode 22 can be achieved by ionimplantation into a predetermined region through use of a mask. It isalso possible to perform the ion implantation into the predeterminedregion, without using the mask, by controlling an ion beam for X-Yscanning.

As described above, the thin film transistor of the present inventionuses an insulating organic molecular film as the gate insulating film,so that the manufacture of this thin film transistor does not involvethe use of such a plasma CVD process as would be needed for theformation of an inorganic insulating film. Accordingly, thesemiconductor layer 14 of good quality can be obtained.

In addition, since the region of the gate insulating film on theopposite side from the semiconductor layer 14 is rendered into aconductive region containing free carbon for use as the gate electrode,its formation can be achieved simply by ion implantation. Therefore, themanufacturing process of the thin film transistor is simple as comparedwith the prior art process which involves two steps of forming ametallic film and selectively etching it away, for the formation of thegate electrode.

Moreover, since the gate electrode is formed in a limited region of thesurface layer of the gate insulating film, the surface of the device isless uneven than in the case where a gate electrode of metal is formedon the gate insulating film. For instance, when the thin film transistorof the present invention is employed in a liquid cyrstal displayelement, wiring such as a gate bus does not much protrude from thesurface of the device. This will lessen the possibility of breakage ofthe wiring, ensuring enhancement of the yield rate of product.

In the case of using the thin film transistor of the present inventionin the liquid crystal display element, the insulating organic molecularlayer 21 is formed all over the surfaces of the underlying layers and isprocessed for orientation. This eliminates the necessity of providing anoriented film and makes the surface of the device less uneven.

The characteristics of the thin film transistor can be controlled asdesired, by selecting the conditions for ion implantation so that thegate electrode 22 may have desired sheet resistance and work function.

It will be apparent that many modifications and variations may beeffected without departing from the scope of the novel concepts of thepresent invention.

What is claimed is:
 1. A thin film transistor comprising:a sourceelectrode and a drain electrode disposed in spaced relation to oneanother on an insulating substrate; a semiconductor layer on thesubstrate between the source and drain electrodes; a gate insulatingfilm in contact with the semiconductor layer; a gate electrode incontact with the gate insulating film; the gate insulating film being aninsulating organic molecular film; the gate electrode being formed in aselected region of the gate insulating film and being a conductive layercontaining free carbon; and said gate electrode being formed on a sideof said gate insulating film and being separated from the semiconductorlayer.
 2. The thin film transistor of claim 1, wherein the insulatingorganic molecular film is of polyimide.
 3. The thin film transistor ofclaim 1, wherein the insulating organic molecular film is of stearicacid.
 4. The thin film transistor of claim 1, wherein the insulatingorganic molecular film is of diacetylene.
 5. The thin film transistor ofclaim 1, wherein the insulating organic molecular film is ofphthalocyanine.
 6. The thin film transistor of claim 1, wherein theinsulating organic molecular film is of anthracene.
 7. The thin filmtransistor of claim 1, wherein the gate electrode and the gateinsulating film are 3000 Å to 1 μm and 1000 to 3000 Å thick,respectively.